Strictly speaking copyright means we should register our work with the copyright authorities in our country but that very few people are following. Most of us are writing a copyright mention in form of a text or words like say '© Umesh' in our pictures or videos.
So, what you require is a video editor in which you can use the edit mode to add the text or logo or your own way of writing some mention of copyright words alerting the people who hack or copy other's data or content. There are many video editor apps available for this purpose and some of them are free while some have premium versions charging a fee for their use.
Some of the popular and common application where such video editing can be performed are KineMaster, AVS Video Editor, Cyberlink PowerDirector, Adobe Premiere Rush, Magisto, VivaVideo, WeVideo, VideoPad Video Editor, Animation Desk, Animoto etc. Each application would have its own way of doing it but there are similarities of the menu items and once you install these program they are very interactive and guide you to do your work effectively.
When you install any of these apps in your mobile and use it then a menu would come on the side of the screen. In most of them you would see a text icon 'T' and clicking this would allow you to write on the screen your intended copyright mention. Though it is not meant for strictly copyright purposes but people use it in that fashion also. This facility in these apps is basically to write text in a text box in a video. Anyway in this case because you are mentioning a copyright sign and alert wordings so this would be visible to user and he would refrain from copying it.
Knowledge is power.